A flat panel display (FPD) typically comprises a glass or a glass like substrate having an array of thin-film transistors formed thereon. To form the array, the substrate is subjected to various high temperature processing steps (e.g., chemical vapor deposition), many of which also require corrosive gases. Typical temperatures for chemical vapor deposition (CVD) processes can reach about 300xc2x0 C. or higher. At these temperatures a substrate is at risk of experiencing non-uniform processing which can degrade display quality. Specifically, in order to process uniformly across its entire surface, a substrate must be flat during processing. Thus, non-uniform processing may occur when the platform that supports the substrate deforms, causing the substrate to deform therewith. Platforms that support substrates during processing are conventionally known as susceptors, and are conventionally made of aluminum because of aluminum""s excellent corrosion resistance when exposed to CVD processing chemicals. The strength of aluminum, however, begins to decline rapidly above 150xc2x0 C. In fact, aluminum begins to soften at 250xc2x0 C. and exhibits xe2x80x9cliquidxe2x80x9d type properties at about 660xc2x0 C. Thus, when exposed to typical CVD processing temperatures, aluminum susceptors may deflect (e.g., bend or droop at the edges when supported at the center as is conventional). Such deflection is further exacerbated by the current trend toward larger and larger display sizes. Current display sizes are about 550xc3x97650 mm, and are projected to reach well above a square meter in the near future. As display size increases, so does susceptor size and the deflection associated therewith.
Accordingly, a susceptor that resists deflection during high temperature processing is needed.
In accordance with a first aspect of the present invention, an inventive susceptor is provided that includes (1) a supporting surface adapted to support a substrate, the supporting surface comprising a first material; and (2) a support frame encased within the first material, the support frame comprising a second material that has a lower coefficient of thermal expansion than the first material. The support frame is offset from a center of the susceptor toward the supporting surface. In one aspect of the invention, the first material is aluminum and the second material is stainless steel, although other materials may be similarly employed as described below.
In accordance with a second aspect of the present invention, an inventive susceptor is provided that includes (1) a supporting surface adapted to support a substrate, the supporting surface comprising a first material; and (2) a support frame encased within the first material, the support frame comprising a second material that has a higher coefficient of thermal expansion than the first material. The support frame is offset from a center of the susceptor away from the supporting surface. In one aspect of the invention, the first material is graphite and the second material is nickel, although other materials may be similarly employed as described below.
Other features and aspects of the present invention will become more fully apparent from the following detailed description of the preferred embodiments, the appended claims and the accompanying drawings.